The present invention relates generally to a method of forming and/or etching silicon oxide during the fabrication of a semiconductor structure, an optical device, or an electromechanical system. Silicon oxide layers formed in accordance with the present invention have particular application in the field of microelectromechanical systems (MEMS).
The term “MEMS” is used to describe a broad class of electromechanical systems having one or more micro and/or nano-sized components. MEMS are characterized in their implementation by the use of micro-machining techniques, such as lithographic and other precision fabrication techniques, to reduce mechanical components to a scale generally comparable to microelectronics. MEMS are further characterized in most instances by the operative assembly, arrangement and/or formation of these miniaturized components on a silicon substrate. Contemporary examples of MEMS-implemented devices include sensors, actuators, gyroscopes, resonators and accelerometers.
The delicate mechanical structures used in conventional MEMS are typically sealed in a chamber. The chamber may be formed using a hermetically sealed metal container or by the bonding of a semiconductor or glass-like substrate having a chamber to house, accommodate or cover the mechanical structure. However, it can be difficult to cost effectively integrate MEMS that employ a hermetically sealed metal container or a bonded semiconductor or glass-like substrate to protect the mechanical structures on the same substrate with high performance integrated circuitry. In this regard, the additional processing required to integrate the high performance integrated circuitry tends to either damage or destroy the mechanical structures.
Another technique for forming a chamber to protect the delicate mechanical structures employs micromachining techniques. Using this approach, a mechanical structure is encapsulated in a chamber using a conventional oxide (SiO2) deposited or formed using conventional techniques, such as oxidation using low temperature techniques (LTO), tetraethoxysilane (TEOS), or the like. Thereafter, this combination is enclosed under a silicon layer or other material, and the oxide removed. When implementing this technique, the mechanical structure is encapsulated prior to packaging and/or integration with integrated circuitry.
Such oxides often exhibits high tensile stress, particularly at deposition corners or steps. Further, such oxides are often formed or deposited in a manner that provides poor coverage of the underlying surface(s). These shortcomings may impact the integrity and/or performance of the MEMS.
Moreover, the removal of conventional oxides during the formation of MEMS may produce an etch residue on the mechanical structures during the encapsulation process. This etch residue may impact the integrity of the mechanical structures and, as such, the performance or operating characteristics of the MEMS (for example, the operating characteristics of a resonator).
In practice, MEMS formed in accordance with conventional techniques often suffer from significant process-induced performance variations or outright component failure. To a great extent, inadequacies associated with the formation and/or removal of oxide layers account for many of these failures and undesired performance variations.
First, many conventional oxide layers crack during MEMS fabrication, and/or damage or crack the MEMS structure on which they are deposited. Cracking occurs during deposition of the oxide and/or during high temperature processing steps applied to the MEMS following deposition of an oxide layer. These high temperature processes may range in temperature from 800° to 1200° C. Too avoid cracking the internal stress of an oxide layer must be well controlled. Both highly tensile and highly compressive oxide layers are undesirable. Highly tensile oxide layers may deform delicate MEMS components or crack outright. Highly compressive oxide layers may also deform MEMS components or cause tensile cracking in these components or in adjacent materials.
It is well understood that deposited oxides tend to densify or shrink when annealed at high temperature. This tendency creates tensile stress in the deposited oxide. In addition, as fabrication temperature rises, a previously deposited oxide on silicon becomes relatively tensile because the thermal expansion coefficient for the oxide is less than that of the adjacent silicon. Under the combined effect of these tendencies, an oxide layer deposited at moderate temperature may crack or may damage the MEMS structures when subsequently heated to significantly higher temperatures.
In many applications, the etch rate of an oxide used in the formation of a MEMS structure must be well controlled and uniform, from batch to batch, across each batch, and across each substrate. This is particularly true where the formation of a MEMS structure requires that some portion of an oxide be left in place while another portion of the oxide is etched away, and where the extent of the oxide etch determines what portion remains. This condition, often called a ‘timed etch,” requires that the oxide etch at a predetermined, predictable, uniform, and homogenous rate.
In many applications, it is highly desirable for the oxide to be conformal. Conformal deposition of an oxide over a MEMS structure provides uniform coating and fills structural gaps with a minimum of deposited oxide on the upper-most surface of the substrate. Unfortunately, highly conformal, conventional oxides are often susceptible to cracking and often contain materials that result in residue formation when the oxide is subsequently etched.
In many MEMS applications, an oxide must be readily susceptible to etching without producing a residue. Conventional oxides deposited from compounds like TEOS are rich in carbon and other contaminates. Such contaminates promote the development of etch residues. This is particularly true where deposited oxides are subsequently exposed to high temperature. Conventional oxides also tend to form inclusions when exposed to high temperatures. Etch residues, which tend to be silicon in nature, can accumulate to the point where the etching process becomes uneven across an etch surface or impossible. Etch residues can accumulate to the point where device functionality becomes impaired.
The adverse impact of etch residues is highly notable in the context of MEMS fabrication. To a much greater extent than conventional semiconductor fabrications, MEMS fabrications require deep cavity oxide etching. HF-vapor etching is particularly sensitive to the formation of a residue since no “washing” mechanism exists to remove residue, as contrasted to wet etching processes. Unlike many conventional semiconductor fabrications, a wet etching process often cannot be applied to removal of an accumulated residue during some MEMS fabrications. Accordingly, an oxide free from etch residue is particularly important to MEMS fabrications.
The desired thickness for oxide layers used during the formation of MEMS varies considerably. This variation notwithstanding the deposition thickness for an oxide layer must be well controlled. The deposition of an oxide layer may be performed many times during the formation of MEMS. Accordingly, the time required to deposit an oxide layer is a material economics consideration in the determination of a MEMS fabrication process.
In sum, an ideal oxide would deposit and anneal without cracking. It would have low, well controlled stress. It would contain minimal contaminants or inclusions even after being exposed to high temperatures. It would be conformal. It would etch at a predictable, homogeneous rate. It would etch in HF vapor without leaving a residue, especially after being exposed to high temperatures. Finally, it would deposit sufficiently fast to economically form layers of varying, well-controlled thickness.
Unfortunately, many of these desirable properties are at odds one with another. For example, highly conformal, conventional oxides are prone to cracking and often contain contaminants that promote etch residue.
There remains a need for an oxide having many, if not all, of the desirable qualities noted above. Such an oxide would provide particular advantage in the fabrication of MEMS.